A thermodynamic process by which natural gas is liquefied to produce liquefied natural gas (LNG) has been developed to meet various needs including higher efficiency and higher capability since 1970s. In order to meet these needs, that is, in order to increase efficiency and capability of a liquefaction process, various attempts to liquefy natural gas by using different refrigerants or different cycles have been continuously made until now. However, the number of liquefaction processes practically applied is very small.
The ‘propane pre-cooled mixed refrigerant process (or C3/MR process)’ is one of the most widely used liquefaction processes up and running. The basic structure of the C3/MR process is shown in FIG. 19. As shown in FIG. 19, a feed gas is pre-cooled to approximately 238K by a multi-stage of propane (C3) Joule-Thomson (JT) cycle. The pre-cooled feed gas is liquefied to 123K and sub-cooled, through heat exchange with a mixed refrigerant (MR) in a heat exchanger. The foregoing C3/MR process employs a refrigeration cycle adopting a single refrigerant and a refrigeration cycle adopting a mixed refrigerant, but this causes the liquefaction process to be complicated and the liquefaction system to be difficult to operate.
Another successful liquefaction process up and running is by ‘Conoco Phillips’ Company, and is based on a cascade process. As conceptually shown in FIG. 20, the liquefaction process by the ‘Conoco Phillips’ Company is composed of three Joule-Thomson cycles using methane (C1), ethylene (C2), and propane (C3), which are pure-component refrigerants. Since this liquefaction process does not use a mixed refrigerant, the operation of the liquefaction process is stable, simple, and reliable. However, a compressor, a heat exchanger, and the like are needed for each of three cycles, and thus the size of the liquefaction system needs to be increased.
Still another successful liquefaction process up and running is the ‘single mixed refrigerant process (or an SMR Process)’. The basic structure of the SMR process is shown in FIG. 21. As shown in FIG. 21, the feed gas is liquefied through heat exchange with a mixed refrigerant in a heat exchange region. In order to achieve this, a single closed-loop refrigeration cycle adopting a mixed refrigerant is used in the SMR process. In this refrigeration cycle, the mixed refrigerant is compressed and cooled, and the mixed refrigerant is condensed through heat exchange in the heat exchange region, and then expanded. The expanded refrigerant again flows into the heat exchange region, to condense the pre-cooled mixed refrigerant and liquefy the feed gas. This SMR process has a simple structure and thus a compact system, but efficiency of the liquefaction process may not be unfavorable.